The present invention relates to a thermal transfer printing medium capable of forming dear print images on a paper sheet of poor surface smoothness (hereinafter referred to as "rough paper").
Conventional thermal transfer printing media of this type are each provided with a bridgingly transferable ink layer. Such an ink layer has a high melt viscosity which is capable of being transferred as bridging over depressed portions of a rough paper while adhering not to the depressed portions but only to projecting portions thereof.
Such a bridgingly transferable ink of a high melt viscosity, however, is poor in permeating property with respect to paper. In transfer printing on a rough paper in particular, the ink adheres only to projecting portions (compared to depressed portions) of the rough paper; therefore, the poor penetrability of the ink results in print images of significantly degraded fixability, which leads to poor abrasion resistance or the like.
It is conceivable, as means for improving the fixability of print images, that the penetrability of ink is enhanced by using a higher printing energy. This, however, results in a difficulty in high-speed printing and wasteful energy consumption.
To maintain the printing energy to a certain level, the ink layer has to be incorporated with a material of good penetrability. The incorporation of such a material reduces the melt viscosity of the ink layer, thus leading to the ink layer exhibiting unsatisfactory bridging transferability.
Thus, the bridging transferability is inconsistent with the fixability if the printing energy is suppressed.
In view of the foregoing, an object of the present invention is to provide a thermal transfer printing medium which assures print images of improved fixability while maintaining satisfactory bridging transferability, thereby enabling the formation of clear print images of satisfactory fastness free of void or drop out portion even on a rough paper with less printing energy.
This and other objects of the present invention will be apparent from the following detailed description.